Controller integrated circuits (IC) for voltage regulators receive a voltage feedback signal corresponding to the output voltage of the regulator and, based on the feedback signal, control the switching of a switching transistor to regulate the output voltage to be a target voltage. The user typically selects values in a simple resistor divider that divides the output voltage so that, when the target output voltage is generated, the node of the divider produces a feedback voltage equal to a fixed reference voltage of, for example, 1 volt. The feedback voltage is connected to one input of an error amplifier, and the other input of the error amplifier receives the fixed reference voltage. The regulator maintains the output voltage such that the divided voltage is equal to the reference voltage.
The switch is typically connected to an output circuit (external to the controller IC) comprising various configurations of an inductor, a smoothing output capacitor, and a diode (or synchronous rectifier) to generate the output voltage. The specific output circuit configuration determines whether the regulator is a buck regulator, a boost regulator, a buck-boost regulator, a positive-to-negative voltage regulator, or other type of regulator. The output circuit is typically designed by the user for a particular application, and the user can control different types of regulators using the same model controller IC.
Some applications may require the voltage regulator to generate a negative output voltage using a positive input voltage, where the operating voltage for all components in the controller IC is a positive voltage. Since the negative output voltage will generate a negative feedback voltage (a divided output voltage), there must be some circuitry in the controller IC that converts the negative feedback voltage to a positive feedback voltage in order for the downstream circuitry to process the feedback voltage (since the downstream components are powered by only a positive operating voltage). Such a voltage regulator outputs a negative voltage (and feedback voltage) with a ripple that is phase-inverted with the control signal for the switching transistor, since reducing the switch duty cycle causes a ramp-up in output voltage. For proper processing of the feedback voltage, the ripple phase must be inverted.
A level shifter that converts a negative feedback voltage to a positive feedback voltage can be used, but such a level shifter has various drawbacks, such as the ability to only accurately process a narrow range of signals. Further, the feedback ripple voltage still needs to be phase-inverted by other circuitry. Such drawbacks require more complicated downstream circuity to compensate for the reduced range of signals and the phase inversion.
A conventional inverting amplifier can also be used to convert a negative feedback voltage to a positive feedback voltage, inherently phase-inverting the feedback voltage. Such a conventional inverting amplifier can process a wide range of input signals. However, such conventional inverting amplifiers have a low input impedance, which places a load on the resistor divider used to create the feedback signal. Such a load thus distorts the magnitude of the feedback signal which, in turn, distorts the output voltage.
FIG. 1 is an example of a conventional inverting amplifier. If we assume the feedback voltage VFB is a negative voltage, the differential amplifier 10 uses feedback through the resistor R2 to cause both of its inputs to be at ground voltage. Since the inverting input terminal of the amplifier 10 is made to be at ground voltage, and assuming the resistors R1 and R2 have the same value, the inverted output voltage VFB_INV must be a positive voltage having an absolute value equal to VFB. Since the resistor R1 is between the feedback voltage VFB and ground voltage, the resistor R1 forms a load at the node of a resistor divider (not shown) used to generate the feedback voltage VFB. Since the user selects the resistor divider values to generate a target feedback voltage, such as 1 volt, when the output voltage is at the target voltage, the added load of the resistor R1 on the resistor divider will distort the feedback voltage magnitude and thereby distort the output voltage. Further, the absolute values of resistors in such an inverting amplifier vary somewhat from lot to lot, so the true loading of the resistor divider in the feedback network would be unpredictable and vary from lot to lot.
What is needed is an inverting amplifier that can receive a negative feedback voltage in a voltage regulator and convert the negative feedback voltage to a positive feedback voltage while not adding any significant load to the resistor divider feedback circuit.